By measuring a terminal voltage of a battery part when charge/discharge is stopped, an open-circuit voltage of the battery part can be detected. FIG. 21 illustrates an example of an equivalent circuit of a battery part 900 composed of a secondary battery. It can be considered that the battery part 900 is equivalent to a serial connection circuit of an ideal voltage source 901 in which internal resistance is zero and an impedance circuit 902. An output voltage of the voltage source 901 corresponds to the open-circuit voltage. In the impedance circuit 902, not only a resistance component but also a capacitance component is included. Thus, since a voltage corresponding to a stored charge of the capacitance component is added to the terminal voltage for some time after charge or discharge is stopped, the terminal voltage does not accurately indicate the open-circuit voltage. Therefore, in a conventional method MTCNV, in order to recognize the open-circuit voltage, the terminal voltage is measured after waiting for the terminal voltage to be sufficiently stabilized after the charge or discharge is stopped (for instance, see Patent Literature 1 below). Though it depends on the structure of the battery part 900, generally an attenuation time constant of the stored charge of the capacitance component inside the impedance circuit 902 is about one hour.